Implemented rewindless Fiber without Boost (using minicoro)

src/common/CMakeLists.txt

@@ -261,7 +261,7 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
   )
 endif()
 
-target_link_libraries(common PUBLIC Boost::context Boost::headers fmt::fmt microprofile stb::headers Threads::Threads)
+target_link_libraries(common PUBLIC Boost::headers fmt::fmt microprofile stb::headers Threads::Threads)
 target_link_libraries(common PRIVATE lz4::lz4 zstd::zstd LLVM::Demangle)
 
 if (ANDROID)

src/common/fiber.cpp

@@ -5,87 +5,47 @@
 
 #include "common/assert.h"
 #include "common/fiber.h"
-#include "common/virtual_buffer.h"
-
-#include <boost/context/detail/fcontext.hpp>
+#define MINICORO_IMPL
+#include "common/minicoro.h"
 
 namespace Common {
 
-constexpr std::size_t default_stack_size = 512 * 1024;
-
 struct Fiber::FiberImpl {
-    FiberImpl() : stack{default_stack_size}, rewind_stack{default_stack_size} {}
-
-    VirtualBuffer<u8> stack;
-    VirtualBuffer<u8> rewind_stack;
+    FiberImpl() {}
 
     std::mutex guard;
-    std::function<void()> entry_point;
-    std::function<void()> rewind_point;
-    std::shared_ptr<Fiber> previous_fiber;
-    bool is_thread_fiber{};
     bool released{};
+    bool is_thread_fiber{};
+    Fiber *next_fiber{};
+    Fiber **next_fiber_ptr;
+    std::function<void()> entry_point;
 
-    u8* stack_limit{};
-    u8* rewind_stack_limit{};
-    boost::context::detail::fcontext_t context{};
-    boost::context::detail::fcontext_t rewind_context{};
+    mco_coro *context;
 };
 
-void Fiber::SetRewindPoint(std::function<void()>&& rewind_func) {
-    impl->rewind_point = std::move(rewind_func);
-}
-
-void Fiber::Start(boost::context::detail::transfer_t& transfer) {
-    ASSERT(impl->previous_fiber != nullptr);
-    impl->previous_fiber->impl->context = transfer.fctx;
-    impl->previous_fiber->impl->guard.unlock();
-    impl->previous_fiber.reset();
-    impl->entry_point();
-    UNREACHABLE();
-}
-
-void Fiber::OnRewind([[maybe_unused]] boost::context::detail::transfer_t& transfer) {
-    ASSERT(impl->context != nullptr);
-    impl->context = impl->rewind_context;
-    impl->rewind_context = nullptr;
-    u8* tmp = impl->stack_limit;
-    impl->stack_limit = impl->rewind_stack_limit;
-    impl->rewind_stack_limit = tmp;
-    impl->rewind_point();
-    UNREACHABLE();
-}
-
-void Fiber::FiberStartFunc(boost::context::detail::transfer_t transfer) {
-    auto* fiber = static_cast<Fiber*>(transfer.data);
-    fiber->Start(transfer);
-}
-
-void Fiber::RewindStartFunc(boost::context::detail::transfer_t transfer) {
-    auto* fiber = static_cast<Fiber*>(transfer.data);
-    fiber->OnRewind(transfer);
+Fiber::Fiber() : impl{std::make_unique<FiberImpl>()} {
+    impl->is_thread_fiber = true;
 }
 
 Fiber::Fiber(std::function<void()>&& entry_point_func) : impl{std::make_unique<FiberImpl>()} {
     impl->entry_point = std::move(entry_point_func);
-    impl->stack_limit = impl->stack.data();
-    impl->rewind_stack_limit = impl->rewind_stack.data();
-    u8* stack_base = impl->stack_limit + default_stack_size;
-    impl->context =
-        boost::context::detail::make_fcontext(stack_base, impl->stack.size(), FiberStartFunc);
+    auto desc = mco_desc_init([] (mco_coro *coro) {
+        reinterpret_cast<Fiber*>(coro->user_data)->impl->entry_point();
+    }, 0);
+    desc.user_data = this;
+    mco_result res = mco_create(&impl->context, &desc);
+    ASSERT(res == MCO_SUCCESS);
 }
 
-Fiber::Fiber() : impl{std::make_unique<FiberImpl>()} {}
-
 Fiber::~Fiber() {
     if (impl->released) {
         return;
     }
-    // Make sure the Fiber is not being used
-    const bool locked = impl->guard.try_lock();
-    ASSERT_MSG(locked, "Destroying a fiber that's still running");
-    if (locked) {
-        impl->guard.unlock();
+    DestroyPre();
+    if (impl->is_thread_fiber) {
+        DestroyThreadFiber();
+    } else {
+        DestroyWorkFiber();
     }
 }
 
@@ -94,42 +54,66 @@ void Fiber::Exit() {
     if (!impl->is_thread_fiber) {
         return;
     }
-    impl->guard.unlock();
+    DestroyPre();
+    DestroyThreadFiber();
+}
+
+void Fiber::DestroyPre() {
+    // Make sure the Fiber is not being used
+    const bool locked = impl->guard.try_lock();
+    ASSERT_MSG(locked, "Destroying a fiber that's still running");
+    if (locked) {
+        impl->guard.unlock();
+    }
     impl->released = true;
 }
 
-void Fiber::Rewind() {
-    ASSERT(impl->rewind_point);
-    ASSERT(impl->rewind_context == nullptr);
-    u8* stack_base = impl->rewind_stack_limit + default_stack_size;
-    impl->rewind_context =
-        boost::context::detail::make_fcontext(stack_base, impl->stack.size(), RewindStartFunc);
-    boost::context::detail::jump_fcontext(impl->rewind_context, this);
+void Fiber::DestroyWorkFiber() {
+    mco_result res = mco_destroy(impl->context);
+    ASSERT(res == MCO_SUCCESS);
+}
+
+void Fiber::DestroyThreadFiber() {
+    if (*impl->next_fiber_ptr) {
+        *impl->next_fiber_ptr = nullptr;
+    }
 }
 
 void Fiber::YieldTo(std::weak_ptr<Fiber> weak_from, Fiber& to) {
-    to.impl->guard.lock();
-    to.impl->previous_fiber = weak_from.lock();
-
-    auto transfer = boost::context::detail::jump_fcontext(to.impl->context, &to);
-
-    // "from" might no longer be valid if the thread was killed
     if (auto from = weak_from.lock()) {
-        if (from->impl->previous_fiber == nullptr) {
-            ASSERT_MSG(false, "previous_fiber is nullptr!");
-            return;
+        if (!from->impl->is_thread_fiber) {
+            // Set next fiber
+            from->impl->next_fiber = &to;
+            // Yield from thread
+            if (!from->impl->released) {
+                from->impl->guard.unlock();
+                mco_yield(from->impl->context);
+            }
+        } else {
+            from->impl->guard.lock();
+            // Keep running next fiber until they've ran out
+            auto& next_fiber_ptr = from->impl->next_fiber_ptr;
+            next_fiber_ptr = &from->impl->next_fiber;
+            *next_fiber_ptr = &to;
+            for ([[maybe_unused]] unsigned round = 0; *next_fiber_ptr; round++) {
+                auto next = *next_fiber_ptr;
+                *next_fiber_ptr = nullptr;
+                next_fiber_ptr = &next->impl->next_fiber;
+                // Stop if new thread is thread fiber
+                if (next->impl->is_thread_fiber)
+                    break;
+                // Resume new thread
+                next->impl->guard.lock();
+                mco_result res = mco_resume(next->impl->context);
+                ASSERT(res == MCO_SUCCESS);
+            }
+            from->impl->guard.unlock();
         }
-        from->impl->previous_fiber->impl->context = transfer.fctx;
-        from->impl->previous_fiber->impl->guard.unlock();
-        from->impl->previous_fiber.reset();
     }
 }
 
 std::shared_ptr<Fiber> Fiber::ThreadToFiber() {
-    std::shared_ptr<Fiber> fiber = std::shared_ptr<Fiber>{new Fiber()};
-    fiber->impl->guard.lock();
-    fiber->impl->is_thread_fiber = true;
-    return fiber;
+    return std::shared_ptr<Fiber>{new Fiber()};
 }
 
 } // namespace Common

src/common/fiber.h

@@ -6,9 +6,7 @@
 #include <functional>
 #include <memory>
 
-namespace boost::context::detail {
-struct transfer_t;
-}
+#include "common/minicoro.h"
 
 namespace Common {
 
@@ -38,25 +36,18 @@ public:
     Fiber(Fiber&&) = default;
     Fiber& operator=(Fiber&&) = default;
 
-    /// Yields control from Fiber 'from' to Fiber 'to'
-    /// Fiber 'from' must be the currently running fiber.
     static void YieldTo(std::weak_ptr<Fiber> weak_from, Fiber& to);
     [[nodiscard]] static std::shared_ptr<Fiber> ThreadToFiber();
 
-    void SetRewindPoint(std::function<void()>&& rewind_func);
-
-    void Rewind();
-
     /// Only call from main thread's fiber
     void Exit();
 
 private:
     Fiber();
 
-    void OnRewind(boost::context::detail::transfer_t& transfer);
-    void Start(boost::context::detail::transfer_t& transfer);
-    static void FiberStartFunc(boost::context::detail::transfer_t transfer);
-    static void RewindStartFunc(boost::context::detail::transfer_t transfer);
+    void DestroyPre();
+    void DestroyWorkFiber();
+    void DestroyThreadFiber();
 
     struct FiberImpl;
     std::unique_ptr<FiberImpl> impl;

src/tests/common/fibers.cpp

@@ -271,43 +271,4 @@ TEST_CASE("Fibers::StartRace", "[common]") {
     REQUIRE(test_control.value3 == 1);
 }
 
-class TestControl4;
-
-class TestControl4 {
-public:
-    TestControl4() {
-        fiber1 = std::make_shared<Fiber>([this] { DoWork(); });
-        goal_reached = false;
-        rewinded = false;
-    }
-
-    void Execute() {
-        thread_fiber = Fiber::ThreadToFiber();
-        Fiber::YieldTo(thread_fiber, *fiber1);
-        thread_fiber->Exit();
-    }
-
-    void DoWork() {
-        fiber1->SetRewindPoint([this] { DoWork(); });
-        if (rewinded) {
-            goal_reached = true;
-            Fiber::YieldTo(fiber1, *thread_fiber);
-        }
-        rewinded = true;
-        fiber1->Rewind();
-    }
-
-    std::shared_ptr<Common::Fiber> fiber1;
-    std::shared_ptr<Common::Fiber> thread_fiber;
-    bool goal_reached;
-    bool rewinded;
-};
-
-TEST_CASE("Fibers::Rewind", "[common]") {
-    TestControl4 test_control{};
-    test_control.Execute();
-    REQUIRE(test_control.goal_reached);
-    REQUIRE(test_control.rewinded);
-}
-
 } // namespace Common